This invention relates to reclaiming plastics and particularly to process improvements applicable to such reclamation.
As set forth, for example, in United States Bureau Of Mines Report of Investigations 7955, entitled "Recycling Of Plastics From Urban And Industrial Refuse", 1974, it is known to reclaim waste plastics for refabrication into similar or different products or for pyrolysis to recover monomers used in forming thermoplastic materials, or for incineration where the plastics serve as a high energy fuel source.
Rigid or semi-rigid (i.e. self-supporting) molecularly oriented thermoplastic articles such as sheet, containers in the form of trays, bottles, cups and the like, have been increasingly appearing in the marketplace and represent candidates for reclamation. Such orientation is employed to eliminate brittleness and to improve strength, thereby facilitating in this latter respect, a reduction in plastic without a corresponding loss in strength properties. Generally, such orientation involves stretching the thermoplastic material while within a pre-selected temperature range in plural (usually two) directions during fabrication in order to orderly align the molecules in the direction of stretch, followed by quenching while under stress to lock in the orientation. One category of molecularly oriented articles having major, though not exclusive, application in food and beverage packaging are containers formed of high barrier thermoplastic materials, i.e. those having respective oxygen and carbon dioxide permeabilities of less than 20 and 30 cc./day/100 sq. in./mil./atm. at 73.degree. F., and it is especially desirable to recover such materials in view of the rather substantial cost to synthesize them.
In general, reclaiming usually involves grinding to reduce bulk, segregating the various types of materials from each other before or after such grinding, optionally followed by washing prior to either recycling for refabrication, or pyrolyzing for monomer recovery, or incinerating for energy recovery. However, it has been found that when molecularly oriented materials are ground with conventional size reduction equipment, the bulk of the particles assume a flake or platelet-like shape, wherein the particle thickness is generally significantly less than its width and breadth. In the portions of the article having substantial molecular orientation, the shear developed during grinding also tends to separate the material into layers, which layers, on being reduced in size, assume the form of such platelets. When the platelets are washed, or sorted in the reclaim process by sink-flotation gravity methods using solutions of different specific gravity, significant quantities of such liquids have been found to carry over with the platelet-shaped particles because of capillary action. Such carry-over can result in expensive losses and in substantial deliquifying and drying loads, the latter dictating an increase in size of various deliquifying components in the process train over that necessary for non-molecularly oriented materials. When the plastic being recovered is to be refabricated into new articles, as opposed to being pyrolyzed or incinerated, these platelets tend to create feeding problems to a screw masticating element in a melt processing step.
As set forth in the above-referenced commonly assigned application, the desireability of shrinking-back molecularly oriented articles to reduce bulk and assist in delabeling in a reclaim environment has been practiced in the prior art by the assignee of this application. However, if the oriented thermoplastic material is initially shrunk back to an unoriented state while in its fabricated condition, much fine material is generated thereafter in the grinding step because of the friable nature of the material, which fines can be difficult to process because of the minute dust-like consistency of the particles.